Aircraft canopies protect the pilot from direct exposure to the flight environment while ideally providing the largest possible unobstructed view from the cockpit. In new fighter aircraft, the exterior shapes of the canopy typically include compound curvatures. These compound shapes are imposed by aerodynamic, structural, bird strike safety and electromagnetic signature requirements. In order to satisfy structural and bird strike safety requirements, plastic-based canopies have become quite thick to withstand these stresses. These thick canopies introduce strong refractive optical power and distortion into the view of the outside world. These optical distortions vary with target azimuth, elevation, and the pilot's eye (x, y, z) position. The distortions thus also vary between the left and right eye at any given viewing direction.
Canopy optical distortions interfere with the accuracy of visual cueing, and must be measured and compensated for in positioning of heads-up display (HUD) or helmet mounted display (HMD) symbology. Canopy manufacturing processes introduce additional optical distortions having even higher spatial frequencies than the basic canopy refractive properties. Unfortunately, these additional manufacturing distortions are unique to each canopy, with no two canopies ever exhibiting identical optical distortions. This situation is made worse as even more complex canopies are planned for new aircraft. These canopies are anticipated to have unique optical distortion characteristics. These unique optical characteristics necessitate the design, manufacture and use of mapping fixtures to individually measure canopy optical distortions over the specified field of regard (FOR) and region around design eye for each canopy produced. These variable optical distortions further necessitate algorithms that dynamically position HUD or HMD symbology based on each canopy's set of optical unique distortions and the pilot's moving head position within the 3-D pilot eye volume (PEV).
Previous mapper solutions sequentially project collimated light from a single small target at a series of scripted azimuth and elevation angles through the test canopy and into a camera lens. The camera is located at or near the pilot's design eye location. The camera forms an image of the projected target. Post-processing software calculates the (x, y) position of the target image and the corresponding azimuth/elevation angular deviation. This method gives very good accuracy and repeatability, on the order of 0.1-0.2 milliradians or better, at each particular measurement angle and eye position.
The principle disadvantage of this method is that only a single angular deviation value is measured at a time. The canopy is positioned on the fixture to place the imaging camera at one specific eye position, and the optical deviations of a scripted array of azimuth and elevation angles are sequentially measured at that eye point. The canopy must then be re-positioned on the fixture to the next eye position, and the same script of azimuth and elevation angle measurements repeated. Typically a minimum of five eye positions are measured, one at design eye, one above, one below, and two more at left and right of design eye. Newer fighter aircraft employing wide field-of-regard HMD's require many more eye positions over the 3-D PEV to be measured as well. The final reported optical deviation and symbology position compensation at any given azimuth and elevation is but a single, fixed weighted least squares composite average of the overlapping deviations from the different eye positions. Symbology positions projected on HUD's or HMD's cannot change with head (x, y, z) position, even though canopy optical distortions are continuously changing with eye position. Thus, projected symbology does not overlay exterior imagery over the complete viewing solid angle and PEV to the accuracies required by the new weapons systems.
Furthermore, sequential measurements can take an hour or more just for the relatively small angular solid angle of measurements needed for the HUD, and 8-10 hours or more for larger solid angles such as required for F-16 HMD's. These long measurement times dictate relatively sparse optical deviation data sampling. Sparse sampling reduces the accuracy of post-measurement curve fitting and symbology placement. New systems such as those employed by the F-35 aircraft require that deviation measurements be taken over a very large field of regard (FOR) for a tactical fighter aircraft, and over a very large 3-D PEV. The need for large solid angle coverage and larger 3-D PEV, combined with increased accuracy requirements on symbology placement, essentially renders previous sequential, discrete measurement methods totally impractical. No previous or present optical deviation mapping fixtures have the means to resolve this deficiency.
Therefore a need exists for a means to rapidly measure optical deviations created by an aircraft canopy through many possible 3-D locations of a pilot's eye within the canopy. Additionally, a need exists for a system and method capable of taking measured optical deviations and dynamically applying corrections to these deviations to dynamically align symbology within an HMD field of view to exterior images.